1. Field of the Invention
The present invention relates to a method of forming an epitaxially grown nitride-based compound semiconductor crystal substrate structure with a reduced dislocation density, and a nitride-based compound semiconductor crystal substrate structure with a reduced dislocation density.
2. Description of the Related Art
Gallium nitride-based compound semiconductors are attractive to persons skilled in the art for a wide variety of applications to various semiconductor devices, for example, light emitting diodes such as a blue-color light emitting diode and laser diodes. The gallium nitride-based compound semiconductors include various compound semiconductors including gallium and nitrogen, for example, gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (GaAlN), and the like. The gallium nitride-based compound semiconductors are relatively superior in thermal stability and environmental requirements. For these reasons, the requirements for applications of the gallium nitride-based compound semiconductors to advanced various electron devices have now been on the increase.
It has been known to the persons skilled in the art that it is not easy to realize a desired crystal growth in bulk-form or single-layered form of the nitride-based compound semiconductor crystal, while such a single-layered nitride-based compound semiconductor crystal substrate would be desirable, wherein an entirety of the substrate comprises a single layer of a nitride-based compound semiconductor crystal.
In accordance with the current practices, however, it is generally common to use a single crystal sapphire substrate as a base structure for allowing any available epitaxial growth such as a metal organic vapor phase epitaxy of a nitride-based compound semiconductor crystal layer on the sapphire substrate, such that the substrate comprises the single crystal sapphire base structure and an epitaxially grown nitride-based compound semiconductor crystal layer on the base structure.
The sapphire single crystal is different in lattice constant from the nitride-based compound semiconductor crystal. This lattice-mismatch between sapphire and nitride-based compound semiconductor is so large as making it difficult to realize a desirable epitaxial growth of the nitride-based compound semiconductor crystal directly on the surface of the sapphire single crystal. In order to solve the above issue of difficulty, it was proposed that a nitride-based compound semiconductor buffer layer such as aluminum nitride or gallium nitride serving as a buffer layer is grown at a relatively low temperature on the sapphire single crystal for subsequent desirable epitaxial growth of the nitride-based compound semiconductor crystal directly on the surface of the buffer layer, wherein the buffer layer contributes to relax a lattice-strain of the epitaxially grown nitride-based compound semiconductor crystal due to the lattice-mismatch between sapphire and nitride-based compound semiconductor. This conventional technique is disclosed in Japanese laid-open patent publication No. 63-188983, which is incorporated herein as reference.
This relaxation by the buffer layer would allow the epitaxial growth of the nitride-based compound semiconductor crystal, but still be incapable of removing dislocations from the epitaxially grown nitride-based compound semiconductor crystal, wherein the density of such dislocations is likely to be, for example, in the order of about 1E9 to 1E10 cm−2. Such dislocation density range would be still higher than a desirable low dislocation density range for practical applications to a variety of gallium nitride-based compound semiconductor devices such as laser diodes and light emitting diodes.
In recent years, some techniques for reducing the density of crystal defects and dislocations of the epitaxially grown gallium nitride crystal over the sapphire single crystal were reported, for example, an epitaxial lateral overgrowth as disclosed in Applied Physics Letter 71, (18) 2638 (1997), incorporated herein as a reference, and a facet-initiated epitaxial lateral overgrowth as disclosed in Japan Journal of Applied Physics 38, L184 (1999), incorporated herein as another reference, as well as a pendio-epitaxy as disclosed in MRS Internet Journal Nitride Semiconductor Res. 4S1, G3, 38 (1999), incorporated herein as still another reference.
In accordance with those growth techniques, a patterned SiO2 mask is disposed on a surface of a gallium nitride base layer epitaxially grown over the sapphire substrate, for subsequent selective epitaxial growth of the gallium nitride layer through openings of the patterned SiO2 mask from exposed surface regions of the gallium nitride base layer, wherein a lateral growth of the gallium nitride crystal over the patterned SiO2mask is caused with changing the epitaxial growth direction of the gallium nitride crystal, so that this change in the epitaxial growth direction contributes to discontinue propagation of the dislocations through openings of the patterned SiO2 mask from exposed surface regions of the gallium nitride base layer. The further development made of those growth techniques results in somewhat effective reductions in the dislocation density into the order of approximately 1E7 cm−2. An example of those growth techniques is also disclosed in Japanese laid-open patent publication No. 10-312971, incorporated herein as yet another reference.
The above-described growth techniques utilizing the patterned SiO2 mask, of course, need additional processes for forming the patterned SiO2 mask on the surface of the gallium nitride base layer epitaxially grown over the sapphire substrate. Those additional processes may, for example, be as follows. An SiO2 film is deposited on the surface of the gallium nitride base layer by using any available deposition method such as a chemical vapor deposition method. A resist material is applied on the surface of the SiO2 film. The resist film is then exposed and subsequently developed in photo-lithography processes to obtain a resist pattern on the SiO2 film. The SiO2 film is selectively etched by the resist pattern, followed by removal of the resist pattern and cleaning process, thereby to obtain the patterned SiO2 mask. Those sequential processes for forming the patterned SiO2 mask are complicated and time-consuming processes, and also need highly accurate advanced micro-lithography techniques. This provides a restriction to a desirable improvement in the yield and re-productivity of the patterned SiO2 mask.
The above-described growth techniques, utilizing the patterned SiO2 mask, may also include numerous heat treatment processes and cleaning processes, and cause pollution and damage to the substrate due to handling. Notwithstanding, the above-described growth techniques, utilizing the patterned SiO2 mask, result in still higher dislocation density levels than the desired low levels for practical applications to the advanced laser diode and/or light emitting diode. Namely, the above-described growth techniques do not satisfy the actual requirements for the practical applications. This is because the gallium nitride crystal epitaxially grown over the patterned SiO2 mask exhibits a crystal strain generation due to a difference in growth mechanism or direction between over a region covered by the patterned SiO2 mask and another region uncovered thereby. This crystal strain generation causes a tilt of crystal axis of the gallium nitride crystal. This is disclosed in Journal of Crystal Growth 208 (2000) pp. 804–808, incorporated herein as a reference.
In the above circumstances, the developments of a novel method of forming an epitaxially grown nitride-based compound semiconductor crystal substrate structure as well as such a substrate free from the above problems are desirable.